Quaternary ammonium tribromides

Quaternary ammonium tribromides in methanol-dichloromethane did not brominate the furan ring of 3-methyl-8//-cyclohepta[/>]furan-8-one. Instead, the 7-bromo derivative was formed (92BCJ295). 

As these solid agents, some quaternary ammonium tribromides such as pyridinium hydrobromide perbromide (ref. 1), phenyltrimethylammonium tribromide (ref. 2), tetramethylammonium tribromide (ref. 3), and tetrabutyl-ammonium tribromide (ref. 4) have already been reported as mild and selective brominating agents (Fig. 1). 

Similarly, we found that quaternary ammonium tribromides such as TBA Br3 and BTMA Br3 are useful agents for the a-bromination of acetyl derivatives (Fig. 16) (ref. 25). 

There are a number of methods for the oxidation of primary alcohols or ethers to dimeric esters, and secondary alcohols to ketones. We recently also found that quaternary ammonium tribromides, especially BTMA Br3, are useful oxidizing agents for the purpose described above (ref. 31). 

Free halogens are generally inconvenient to use, owing to their toxic and corrosive nature, but can be replaced by quaternary ammonium polyhalides. Quaternary ammonium tribromides are well established [e.g. 1] as solid, readily handled and relatively non-toxic alternatives for electrophilic bromine. More recently, other quaternary ammonium polyhalides have been produced, which together with the tribromides, have wide application as catalysts or in stoichiometric quantities in electrophilic substitution and addition reactions, oxidations, etc. 

Quaternary ammonium tribromides can also be produced in situ from the quaternary ammonium bromide, sodium hypochlorite and sodium bromide and can be used, for example, in electrophilic addition reactions reaction with alkenes and alkynes. 

As an alternative to the use of quaternary ammonium tribromide, A -bromosuccin-imide tetra-n-butylammonium bromide converts alkenes into the dibromoalkanes generally in high yield (>90%) [8]. It is probable that the ammonium tribromide is formed in situ. 

This reaction gives fair-to-good yields of monoorganotin tribromides and trichlorides when quaternary ammonium or phosphonium catalysts are used (149). Better yields are obtained with organic bromides and staimous bromide than with the chlorides. This reaction is also catalyzed by tri alkyl antimony compounds at 100—160°C, bromides are more reactive than chlorides in this preparation (150,151). a,C0-Dihaloalkanes also react in good yield giving CO-haloalkyltin trihaHdes when catalyzed by organoantimony compounds (152). 

Halogenation can be run under milder conditions using more active quaternary ammonium polyhalides such as pyridinium tribromide. The reaction between thiophene and benzyltrimethylammonium tribromide in acetic acid-ZnCl2 provided 2,5-dibromothiophene [6]. Chloro- and iodo-substituted thiophene derivatives may be prepared in the same manner. In comparison, bromination of thiophene employing 2 equivalents of NBS in chloroform gave 2,5-dibromothiophene in 56% yield [7]. 

Halogens are frequently used as oxidation agents and, under two-phase conditions, they can either be employed as ammonium complex halide salts [3], or in the molecular state with or without an added quaternary ammonium catalyst [4]. Stoichiometric amounts of tetra-n-butylammonium tribromide under pH controlled conditions oxidize primary alcohols and low-molecular-weight alkyl ethers to esters, a,cyclic ethers produce lactones [3], and secondary alcohols yield ketones. Benzoins are oxidized to the corresponding benzils (80-90%) by the tribromide salts in acetonitrile in the presence of benzoyl peroxide [5]. 

The addition of bromine to quaternary ammonium bromides to provide the tribromides is most easily achieved in a reaction column with the solid while a stream of air carries the required amount of bromine (Scheme 11). Thus, in the case of tetrabutylammonium bromide (TBAB), a sharp yellow reaction front is obtained while bromine is quantitatively added and the pure bromi-nation agent TBABrg is obtained. Small runs can be quantitatively performed with 0.5 bar bromine vapor and the unground crystals of TBAB [28]. Equally simple is the gas-solid addition of chlorine to triphenylphosphane to give triphenylphosphane dichloride [28]. 

Kajigaeshi, S., Asano, K., Fujisaki, S., Kakinami, T., Okamoto, T. Oxidation using quaternary ammonium poiyhaiides. i. An efficient method for the Hofmann degradation of amides by use of benzyitrimethyiammonium tribromide. Chem. Lett. 1989, 463-464. 

Methyl tribromide. See Bromoform Methyl tributyl ammonium chloride CAS 56375-79-2 EINECS/ELINCS 260-135-8 Synonyms 1-Butanaminium, N,N-dibutyl-N-methyl, chloride N,N-Dibutyl-N-methyl-1-butanaminium chloride MTBCI Tributylmethylammonium chloride Classification Quaternary salt Empirical C13H30N Cl Formula (C4H9)3(CH3NCI Properties Wh. to off-wh. cryst. si. tributylamine odor very sol. in water m.w. 235.84 m.p. 105-108 C... 

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